Gas turbines have been widely used to provide electrical power, usually as a standby for both peak power and reserve power requirements in the utility industry. Gas turbines are preferred because of their rapid starting capability and low capital costs. Conventional gas turbines, however, operate with reduced thermal efficiency due to the high exit temperatures of the exhaust gas stream and the resulting thermal loss. Therefore, a gas turbine is often combined with the heat recovery steam generator to improve overall system efficiency.
The heat recovery steam generator can be employed to drive a steam turbine for power output or to provide process steam in cogeneration cycles. Heat recovery steam generators typically have either a vertical exhaust gas flow or a horizontal exhaust gas flow through arrangements of heat recovery and air pollution control assemblies. The heat recovery assemblies, or heat exchange circuits, conventionally include superheaters, evaporators, economizers and preheaters. In heat recovery steam generators having vertical exhaust gas flow, the exhaust gas stream from the gas turbine flows upward through stacked arrangements of heat recovery assemblies and air pollution control assemblies. These heat recovery assemblies of the heat recovery steam generators having vertical exhaust gas flow employ horizontally oriented heat transfer tubes. The horizontally oriented heat transfer tubes have forced circulation of a heat transfer fluid therethrough. The use of horizontally oriented heat transfer tubes having forced circulation can permit rapid start up of the heat recovery steam generator.
Conventionally, in a heat recovery assembly having horizontal heat transfer tubes, the heat transfer tubes extend through vertical pairs of spaced apart parallel heat transfer tube support plates. The horizontal tubes are arranged in horizontal rows, a conventional heat recovery assembly having many rows. Typically, a heat transfer assembly has more than 20 rows of heat transfer tubes. The heat transfer tube support plates are suspended within the housing. The mechanical load and thermal stresses exerted on the heat transfer tube support plate are in the same vertical direction when a heat recovery assembly with horizontal heat transfer tubes is employed in a heat recovery steam generator with vertical exhaust gas flow. The mechanical stress on the support plates is generally along a vertical line due to the suspended arrangement of the support plates. The thermal gradient and therefore the thermal stresses on the heat transfer tube support plates are generally constant along any given horizontal line, but vary in the vertical direction. The vertical variation in the thermal gradient and therefore the thermal stresses arises from the cooling of the exhaust gas during passage through the heat recovery assembly.
The support plates are free to expand down as the heat recovery assembly heats up due to the suspension of the support plates in the housing. The resulting downward expansion and therefore the thermal stress is in a generally uniform manner. The thermal expansion of the upper portion of the support plate will be less than the thermal expansion of the lower portion of the support plate due to the variation of the thermal gradient along a vertical line. Again, however, the thermal expansion along any given horizontal line is uniform resulting in a uniform downward expansion of the support plate.
Heat recovery steam generators having horizontal exhaust gas flow have vertically upright heat recovery and air pollution control assemblies. The heat transfer tubes of the heat recovery assemblies are vertically oriented and have natural circulation of the heat transfer fluid therethrough. Horizontal exhaust gas flow is particularly preferred for heat recovery steam generators having limitations on height or structure compared to the height or structure typically required for a vertically oriented exhaust gas flow path.
The use of a conventional heat recovery assembly having horizontally oriented heat transfer tubes in a heat recovery steam generator having a horizontal gas flow results in distortion or warpage of the conventional heat transfer tube support plates. The support plate of a conventional heat recovery assembly having horizontal heat transfer tubes is relatively wide, supporting many rows of heat transfer tubes. Typically, a heat recovery assembly has more than 20 rows of heat transfer tubes. The mechanical load on the heat transfer tube support plates is in the vertical direction due to the suspension of the support plates within the housing. The thermal gradient on the support plate is generally constant along a vertical line in contrast to a vertical exhaust gas flow wherein the thermal gradient is generally constant along a horizontal line. In the horizontal exhaust gas arrangement, the thermal gradient varies along any given horizontal line of the support plate as the horizontally flowing exhaust gas is cooled by passage through the heat recovery assembly. As a result, the portion of the support plate in the upstream direction will generally expand vertically downward a greater amount than the support plate portion in the horizontal downstream direction due to the upstream portion having a generally higher temperature. Therefore, the mechanical and thermal stresses within the support plate are perpendicular to each other. The result of the non-parallel arrangement of the mechanical and thermal stresses is the distortion or warpage of the support plate and the potential for failure of the heat transfer tubes.